Lawyers for the Boston Marathon bombing suspect Dzhokhar Tsarnaev are seeking to have multiple charges against him dismissed.

In a court filing late on Friday his lawyers said some charges were repetitive and that the number could sway jurors weighing whether to find him guilty and sentence him to death.

Three people were killed and more than 260 wounded in the 2013 bombing. Tsarnaev is also charged in the killing of a police officer during a shootout days later.

More than half of the 30 federal charges carry a possible death sentence. Experts have said earlier filings suggest the defence may try to save Tsarnaev’s life by arguing he fell under the influence of his older brother, who died in the shootout.

US prosecutors said on Friday that an FBI agent overheard Tsarnaev make a “statement to his detriment” when his sister visited him in prison. Prosecutors did not reveal what Tsarnaev said but they objected to what they called an attempt by Tsarnaev’s lawyers to suppress the statement.

Tsarnaev made the remark when an investigator working for his lawyers accompanied Tsarnaev’s sister to a prison visit, a meeting that was monitored by an FBI agent, prosecutors said. The defence investigator started to explain to Tsarnaev’s sister the rationale behind special restrictions placed on Tsarnaev in prison, prosecutors said. They say Tsarnaev, “despite the presence of an FBI agent and an employee of the federal public defender, was unable to temper his remarks and made a statement to his detriment which was overheard by the agent”.

The government described the conversation in a memo outlining its opposition to a request from Tsarnaev’s lawyers to lift prison restrictions, known as special administrative measures.

Tsarnaev’s lawyers have argued that prison restrictions limit Tsarnaev’s interactions with people helping his defence team. Prosecutors maintain the FBI agent’s presence was permitted by the special administrative measures, which prohibit providing information to people outside the prison.

Tsarnaev’s lawyers say the presence of the FBI agent during prison visits by Tsarnaev’s two sisters “has thwarted the defence ability to develop important mitigation information”.

Prosecutors have argued that the restrictions are necessary in Tsarnaev’s case because of his “commitment to jihad” and his “widespread notoriety”.